Optical filter with stacked layers

Abstract

An optical filter includes a first layer including a first notch filter arranged to attenuate electromagnetic radiation having a first wavelength λ.sub.1 incident normally thereupon. The optical filter includes a second layer including a second notch filter arranged to attenuate electromagnetic radiation having a second wavelength λ.sub.2 incident normally thereupon. The first wavelength λ.sub.1 and the second wavelength λ.sub.2 are different. The second layer is stacked upon the first layer. In use, the first notch filter attenuates the electromagnetic radiation having a predetermined wavelength λ incident thereupon at a first angle of incidence θ.sub.1 and the second notch filter attenuates the electromagnetic radiation having the predetermined wavelength λ incident thereupon at a second angle of incidence θ.sub.2, wherein the first angle of incidence θ.sub.1 and the second angle of incidence θ.sub.2 are different.

Claims

1. An optical filter for a visor, the optical filter comprising: a first layer comprising a first notch filter arranged to attenuate electromagnetic radiation having a first wavelength, incident normally thereupon; and a second layer comprising a second notch filter arranged to attenuate electromagnetic radiation having a second wavelength, incident normally thereupon, wherein the first wavelength and the second wavelength are different; wherein the second layer is stacked above the first layer; and whereby, in use, the first notch filter attenuates electromagnetic radiation having a predetermined wavelength incident thereupon at a first cone angle of incidence, and the second notch filter attenuates the electromagnetic radiation having the predetermined wavelength incident thereupon at a second cone angle of incidence, wherein the first cone angle of incidence is within the second cone angle of incidence, and wherein the second cone angle of incidence is larger than the first cone angle of incidence.

2. The optical filter according to claim 1, wherein the first notch filter is arranged to attenuate electromagnetic radiation having a first wavelength range including the first wavelength; and wherein the second notch filter is arranged to attenuate electromagnetic radiation having a second wavelength range including the second wavelength; whereby, in use, the first notch filter attenuates the electromagnetic radiation having the predetermined wavelength incident thereupon at a first angle of incidence in the first cone angle of incidence, and the second notch filter attenuates the electromagnetic radiation having the predetermined wavelength incident thereupon at a second angle of incidence in the second cone angle of incidence.

3. The optical filter according to claim 2, wherein the first wavelength range and the second wavelength range each include the predetermined wavelength.

4. The optical filter according to claim 2, wherein the first wavelength range or the second wavelength range is at most 30 nm.

5. The optical filter according to claim 1, wherein the first layer comprises a first set of first notch filters, including the first notch filter, arranged to attenuate electromagnetic radiation having respective first wavelengths, including the first wavelength.

6. The optical filter according to claim 1, wherein the second layer comprises a second set of second notch filters, including the second notch filter, arranged to attenuate electromagnetic radiation having respective second wavelengths, including the second wavelength.

7. The optical filter according to claim 1, wherein the first wavelength, the second wavelength, or the predetermined wavelength is in a range from 100 nm to 1100 nm.

8. The optical filter according to claim 1, wherein a difference between the second wavelength and the first wavelength is in a range from 0.1 nm to 150 nm.

9. The optical filter according to claim 1, wherein the first notch filter has a first optical density of at least 2 or wherein the second notch filter has a second optical density of at least 2.

10. The optical filter according to claim 1, wherein the optical filter comprises a conformable optical filter.

11. The optical filter according to claim 1, wherein the second layer is stacked directly upon the first layer.

12. The optical filter according to claim 1, comprising: a third layer comprising a third notch filter arranged to attenuate electromagnetic radiation having a third wavelength, incident normally thereupon, wherein the first wavelength, the second wavelength and the third wavelength are different; wherein the third layer is stacked above the second layer; and whereby, in use, the third notch filter attenuates the electromagnetic radiation having the predetermined wavelength incident thereupon at a third cone angle of incidence, wherein the first cone angle of incidence, the second cone angle of incidence and the third cone angle of incidence are different.

13. A visor or a windshield comprising the optical filter according to claim 1.

14. A method of manufacturing the visor or the windshield according to claim 13, the method comprising: providing the first layer comprising the first notch filter arranged to attenuate electromagnetic radiation having the first wavelength; providing the second layer comprising the second notch filter arranged to attenuate electromagnetic radiation having the second wavelength; stacking the second layer above the first layer, thereby forming the optical filter; and applying the first layer to the visor or the windshield.

15. The method according to claim 14, wherein stacking the second layer above the first layer includes stacking the second layer directly upon the first layer.

16. An optical filter for a visor, the optical filter comprising: a first layer comprising a first notch filter arranged to attenuate electromagnetic radiation having a first wavelength, incident normally thereupon; and a second layer comprising a second notch filter arranged to attenuate electromagnetic radiation having a second wavelength, incident normally thereupon, wherein the first wavelength and the second wavelength are different, and wherein the first wavelength range or the second wavelength range is at most 20 nm; wherein the second layer is stacked above the first layer; and whereby, in use, the first notch filter attenuates electromagnetic radiation having a predetermined wavelength incident thereupon at a first cone angle of incidence, and the second notch filter attenuates the electromagnetic radiation having the predetermined wavelength incident thereupon at a second cone angle of incidence, wherein the first cone angle of incidence is within the second cone angle of incidence, and wherein the second cone angle of incidence is larger than the first cone angle of incidence.

17. The optical filter according to claim 16, wherein the first wavelength range or the second wavelength range is at most 10 nm.

18. An optical filter for a visor, the optical filter comprising: a first layer comprising a first notch filter arranged to attenuate electromagnetic radiation having a first wavelength, incident normally thereupon; and a second layer comprising a second notch filter arranged to attenuate electromagnetic radiation having a second wavelength, incident normally thereupon, wherein the first wavelength and the second wavelength are different, and wherein a difference between the first wavelength and the second wavelength is in a range from 1 nm to 100 nm; wherein the second layer is stacked above the first layer; and whereby, in use, the first notch filter attenuates electromagnetic radiation having a predetermined wavelength incident thereupon at a first cone angle of incidence, and the second notch filter attenuates the electromagnetic radiation having the predetermined wavelength incident thereupon at a second cone angle of incidence, wherein the first cone angle of incidence is within the second cone angle of incidence, and wherein the second cone angle of incidence is larger than the first cone angle of incidence.

19. The optical filter according to claim 18, wherein the first wavelength, the second wavelength, or the predetermined wavelength is in a range from 380 nm to 1100 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

(2) FIG. 1 schematically depicts an optical filter according to an exemplary embodiment;

(3) FIG. 2 schematically depicts a visor according to an exemplary embodiment;

(4) FIG. 3 schematically depicts an optical filter according to an exemplary embodiment;

(5) FIG. 4 schematically depicts a method of providing an optical filter according to an exemplary embodiment;

(6) FIG. 5 schematically depicts transmission characteristics of an optical filter according to an exemplary embodiment;

(7) FIG. 6 schematically depicts transmission characteristics of an optical filter according to an exemplary embodiment; and

(8) FIG. 7 schematically depicts a method of manufacturing according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

(9) FIG. 1 schematically depicts an optical filter 100 according to an exemplary embodiment.

(10) Particularly, the optical filter 100 is for a visor. The optical filter 100 comprises a first layer 101 comprising a first notch filter 102 arranged to attenuate electromagnetic radiation having a first wavelength λ.sub.1 incident normally thereupon. The optical filter 100 comprises a second layer 103 comprising a second notch filter 104 arranged to attenuate electromagnetic radiation having a second wavelength λ.sub.2 incident normally thereupon. The first wavelength λ.sub.1 and the second wavelength λ.sub.2 are different. The second layer 103 is stacked upon the first layer 101. In use, the first notch filter 102 attenuates the electromagnetic radiation having the predetermined wavelength λ incident thereupon at a first angle of incidence θ.sub.1 and the second notch filter attenuates the electromagnetic radiation having the predetermined wavelength λ incident thereupon at a second angle of incidence θ.sub.2, wherein the first angle of incidence θ.sub.1 and the second angle of incidence θ.sub.2 are different.

(11) In this way, the optical filter 100 provides protection from the electromagnetic radiation having the predetermined wavelength λ incident thereupon (i.e. hostile light) at the first angle of incidence θ.sub.1 and the second angle of incidence θ.sub.2 (i.e. within a larger cone angle) since the first notch filter 102 and the second notch filter 104 attenuate the electromagnetic radiation having the predetermined wavelength λ incident thereupon at different first and second angles of incidence, respectively. In this way, the optical filter 100 provides protection from the electromagnetic radiation having the predetermined wavelength λ incident thereupon (i.e. hostile light) wherein a viewing geometry is less constrained since a larger effective cone angle is provided, thereby permitting relative movement of a user and the optical filter 100. The larger effective cone angle comprises two cone angles, one for each notch filter, having a common origin.

(12) Geometry for Red Shift of Filter Calculations

(13) It will be appreciated that θ.sub.1 and θ.sub.2 may vary. For example at the centre point of the two observers θ.sub.1=−θ.sub.2; and when the light is normal to the filter for the first observer θ.sub.1=0.

(14) Expressing the first oblique angle θ in radians, the wavelength of attenuation, for example blocking, is blue shifted according to Equation 1:

(15) $\lambda \left(\theta \right)=\lambda \left(0\right)\sqrt{1-{\left(\frac{\sin \left(\theta \right)}{n_{\mathrm{eff}}}\right)}^2}$

(16) where n.sub.eff is the effective refractive index and λ(θ) is the predetermined wavelength, incident normally to the first notch filter.

(17) Therefore the nominal wavelength needs to be red shifted by a value λ(0)-λ(θ). This value may be different for the two notch filters providing binary protection.

(18) Table 1 shows red shifts calculated from Equation 1 as a function of θ for λ(0)=532 nm and n.sub.eff=1.5.

(19) TABLE-US-00001 TABLE 1 red shifts calculated from Equation 1 as a function of θ for λ(0) = 532 nm and n.sub.eff = 1.5. θ Red shift (nm)  −80° 130.7 −70° 117.3 −60° 97.6 −50° 74.6 −40° 51.3 −30° 30.4 −20° 14.0 −10° 3.6  0° 0.0  10° 3.6  20° 14.0  30° 30.4  40° 51.3  50° 74.6  60° 97.6  70° 117.3  80° 130.7

(20) Consider a flat visor, comprising the optical filter 100, positioned 10 cm in front of the two pupils (i.e. a first pupil and a second pupil), which are separated by 10 cm. At the region of the filter equidistant from the two pupils (i.e. the centre), the angle to both of the pupils is approximately 26.5° and the first notch filter 102 and the second notch filter 104 need to be red shifted by approximately 24 nm. However, if the region is normal to the first pupil, the angle to the second pupil is approximately 45° and the first notch filter 102 for the second pupil needs to be red shifted by approximately 63 nm while the second notch filter 104 for the first pupil is not red shifted. If the region is 10 cm to the left of the first pupil, the angle to this pupil is 45° degrees and the first notch filter 102 needs to be red shifted by approximately 60 nm while the angle to the second pupil is approximately 63.5° so the second notch filter 104 needs to be red shifted by approximately 105 nm. It should be understood that the first notch filter and the second notch filter 104 may be interchanged.

(21) FIG. 2 schematically depicts a visor 10 according to an exemplary embodiment.

(22) Particularly, FIG. 2 shows a helmet having the visor 10, wherein the visor 10 comprises the optical filter 100, as described above with reference to FIG. 1, provided thereacross. In this way, binary (i.e. two eye) protection is better provided for a user of the helmet.

(23) FIG. 3 schematically depicts the optical filter 100 according to an exemplary embodiment.

(24) The first notch filter 102 is provided as the layer 101 applied to a first face of a substrate 110 to provide at least a part of the optical filter 100 adapted for mitigating laser threats such as dazzle. The substrate 110 is substantially transmissive of visible light (for example it may have a visible light transmission (VLT %) of around 90% of normally incident light) and may be formed for example from a glass or a plastics material such as polycarbonate.

(25) The first notch filter 101 is an interference filter formed by holographically exposing a photosensitive film with a plurality of lasers having a set of predetermined wavelengths within a selected wavelength band of bandwidth 10 nm or less.

(26) Similarly, the second notch filter 104 is provided as the layer 103 applied to (i.e. stacked upon) the first layer 101 to provide the optical filter 100. Similarly, the second notch filter 103 is an interference filter formed as described above with respect to the first notch filter.

(27) Conformable photosensitive (e.g. polymeric) films for use in exemplary embodiments of the present invention will be known to a person skilled in the art, and the present invention is not necessarily intended to be limited in this regard. Such photosensitive polymeric films are provided having varying degrees of inherent visible light transmission (VLT), ranging from less than 70% (and possibly, therefore, having a coloured tinge) up to 99% or more (and being substantially colourless and transparent). In respect of the present invention, suffice it to say that a photosensitive flexible/conformable (e.g. polymeric) film is selected having an inherent VLT of, for example, at least 85%. The film typically has a thickness of 1 to 100 micrometers. Thinner, currently known, films may not achieve useful optical densities. Indeed, in respect of currently known photosensitive polymeric films, the degree to which a selected radiation wavelength can be blocked (i.e. the effectiveness of a filter region formed therein) is determined by the thickness and refractive modulation index of the film and, also, by the optical design. Thus, the filter region thickness is ideally matched to the application and the potential power of the source from which protection is required (which may be dictated, at least to some extent, by the minimum distance from the target platform the laser threat may realistically be located and this, in turn, is dictated by application). In general, thicker films and films with higher refractive modulation indices would be selected if it were required to provide protection from higher power radiation sources or to provide greater angular coverage, but this might then have a detrimental effect on the inherent VLT of the film, so a balance is selected to meet the needs of a specific application. Two or more first notch filters 101 may be stacked to improve attenuation.

(28) Thus, once the film has been selected, the required holographic exposure thereof is effected to form the filter regions of a required notch filter region to be provided thereon, as described below with reference to FIG. 4.

(29) FIG. 4 schematically depicts a method of providing the optical filter 100 according to an exemplary embodiment.

(30) The first notch filter 101 and the second notch filter 103 may be provided generally similarly, successively in separate holographic exposures or in the same holographic exposure.

(31) Particularly, as shown in FIG. 4, distinct filter regions defining a notch filter region of a predetermined bandwidth (for example 5-10 nm) may be formed by exposing the film to the intersection of two counter propagating laser beams for each of a set of laser wavelengths within the selected wavelength band having a selected spectral bandwidth. Each laser 1000 (of a wavelength within the selected spectral bandwidth) produces a laser beam 120 which is controlled by a shutter 140. The laser beam 120 is directed by a mirror 160 into a beam splitter 180 wherein the beam is divided into equal beam segments 200. Each beam segment 200 passes through a microscope objective 220 and is then reflected by a respective mirror 360 onto the photosensitive polymer film 320. Other optical devices (not shown) may be provided between the microscope objective 220 and the mirror 360 to, for example, focus or diverge the respective beam segments 200, as required. Furthermore, masking or other limiting techniques may be utilised to limit the extent or thickness to which the film is exposed to the beam segments 200, as will be understood by a person skilled in the art. As a specific (non limiting) example, if it is required to provide a notch filter region of bandwidth 5 nm around 520 nm, then a plurality of lasers 1000 may be used to produce the notch filter region of (purely by way of example) 517.5 nm, 518 nm, 518.5 nm, 519 nm, 519.5 nm, 520 nm, 520.5 nm, 521 nm, 521.5 nm, 522 nm and 522.5 nm. The above-described exposure process may be performed consecutively for each of these laser wavelengths or, in other exemplary embodiments, the exposures may be performed substantially simultaneously. Other apparatus for forming a holographic filter region at each specified wavelength is known and could, alternatively, be used.

(32) Once the exposure process has been completed, the resultant hologram can be fixed by, for example, a bleaching process.

(33) FIG. 5 schematically depicts transmission characteristics of the optical filter 100 according to an exemplary embodiment.

(34) Particularly, FIG. 5 shows the transmission characteristics (which may alternatively be referred to as the transfer function) of visible electromagnetic radiation incident on the first notch filter 101. The transmission intensity relative to incident radiation intensity is shown on the y-axis and the wavelength of the incident radiation is shown on the x-axis.

(35) As can be seen on the plot, across the range of wavelengths the intensity of the transmitted radiation is close to 100% of that which is incident. In general, a VLT % of 90% would be acceptable if 100% were not feasible.

(36) There are three distinct notches in the transmission characteristic associated with three wavelength bands. These are in particular a 10 nm band centred on 455 nm, a 10 nm band centred on 532 nm and a 10 nm band centred on 650 nm. In general any three notches from the group consisting of 405 nm, 455 nm, 520 nm, 532 nm, and 650 nm may be selected. Further, notches may be chosen to coincide with any expected laser threat wavelength. Still further, the bandwidth may be 5 nm.

(37) At the centre of each of these bands, the intensity of the transmitted radiation is at a minimum and has an optical density of approximately 3, which is equivalent to 0.1% of the initially incident radiation.

(38) FIG. 6 schematically depicts transmission characteristics of an optical filter according to an exemplary embodiment.

(39) Particularly, FIG. 6 shows the measured transmission characteristics of visible electromagnetic radiation incident on the first notch filter 101. The transmission intensity relative to incident radiation intensity is shown on the y-axis and the wavelength of the incident radiation is shown on the x-axis, as described with reference to FIG. 5.

(40) FIG. 7 schematically depicts a method of manufacturing according to an exemplary embodiment.

(41) FIG. 7 schematically depicts a method of manufacturing the visor 10 or a windshield according to the second aspect.

(42) At S701, the first layer 101, comprising the first notch filter 102 arranged to attenuate electromagnetic radiation having the first wavelength, is provided.

(43) The first notch filter 102 may be provided as described with reference to FIGS. 3 and/or 4, for example.

(44) At S702, the second layer 103, comprising the second notch filter 104 arranged to attenuate electromagnetic radiation having a second wavelength, is provided, wherein the first wavelength and the second wavelength are different.

(45) The second notch filter 104 may be provided as described with reference to FIGS. 3 and/or 4, for example.

(46) At S703, the second layer 103 is stacked upon the first layer 101, thereby forming the optical filter 100.

(47) At S704, the first layer 101 is applied to the visor 10 or the windshield.

(48) It should be understood that an order of the steps S701 to S704 may be varied. For example, the first layer 101 may be applied to the visor 10 or the windshield and subsequently, the second layer 103 may be stacked upon the first layer 101, thereby forming the optical filter 100 on the visor 10 or the windshield.

(49) Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.

(50) Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

(51) All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.

(52) Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(53) The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.